Carbon nanotubes are known to exist in single wall and multi-wall configurations. Each configuration provides certain benefits. Single wall nanotubes are preferred for electronic applications due to the low occurrence of structural anomalies. However, multi-wall nanotubes are generally lower in cost and will provide satisfactory performance in electronic applications if the number of walls forming the nanotubes can be controlled. Unfortunately, current methods for producing multi-wall carbon nanotubes lack the ability to control the resulting number of walls in the structure in the resulting nanotubes. As a result, currently produced multi-wall carbon nanotubes generally range in diameter from about 3 to 35 nm and comprise 3 to 40 concentric graphene layers, i.e. walls. The layers are coaxially arranged cylinders of carbon atoms having an interlayer distance of about 0.37 nm. This wide distribution range in walls and external diameter size limits the value of multi-wall nanotubes for electrical conductivity applications, thermal conductivity applications and mechanical reinforcement applications.
In contrast, multi-wall nanotubes, having a relatively narrow distribution range of walls and external diameters, will provide electrical conductivity characteristics approaching those of single wall nanotubes. Additionally, multi-wall nanotubes will provide such improvement at a lower cost. Further, multi-wall nanotubes batches having narrow distributions of wall numbers and external diameters will provide enhanced thermal conductivity and mechanical strength when compared to batches having wide distribution ranges.
While one might consider simply isolating a narrow distribution of multi-wall carbon nanotubes from the wide distribution ranges presently manufactured, technology does not exist for carrying out this task. Thus, the currently available multi-wall nanotubes are provided solely in batches or lots having the undesirable wide distributions of walls and external diameters.
As discussed in detail below, the present invention provides batches of multi-wall nanotubes having narrow distribution ranges of walls and diameters. When incorporated into thermoplastics the narrow distribution range batches provide electrical conductivity characteristics which rival single wall nanotubes and are significantly improved over currently available batches of multi-wall nanotubes. The current invention further provides catalysts and methods for preparing batches of multi-wall carbon nanotubes having narrow distribution ranges of walls and external diameters.